Speaker
Description
Intermediate-mass black holes (IMBHs), with masses between $10^2$ and $10^5$$M_\odot$, may represent a missing link between stellar and supermassive black holes, yet few compelling candidates are known. In the Milky Way globular cluster Omega Centauri ($\omega$ Cen), Häberle et al. (2024) recently identified several stars near the cluster centre with projected velocities exceeding the escape velocity, implying the presence of a central compact object with a mass $M_\bullet$ ≳ $8\times$ $10^3 M_\odot$ and making it a strong IMBH candidate. We follow up on previous work in which we jointly modelled the orbits of the five most reliably measured fast-moving stars. Simultaneously fitting Keplerian orbits to their sky positions, proper motions, and line-of-sight velocities yielded an IMBH mass of $M_\bullet = (6.4^{+0.8}_{-0.7}) \times 10^4$ $M_\odot$. We predict that sky-plane accelerations for the three innermost stars will become detectable ($>3\sigma$) by 2030, which can confirm an IMBH and constrain its properties with greater precision. Our inferred mass agrees with Monte Carlo $N$-body simulations of $\omega$ Cen (González Prieto et al. 2025), which grow IMBHs to around ~ $50{,}000$$M_\odot$ over 12 Gyr from initial black hole seeds of $500$ - $5000$$M_\odot$. However, the formation mechanisms for such massive seeds early in the cluster's evolution are an open question. Using rapid cluster-evolution models motivated by the initial conditions of $\omega$ Cen, we identify viable seed-formation pathways through runaway stellar collisions and hierarchical black hole mergers.
